Adaptive current limiter for wireless modem

ABSTRACT

Method and apparatus for preventing current overdraw by a wireless modem sinking current from a host power supply of a host device (e.g. laptop computer or personal digital assistant (PDA)). The methods and apparatuses are applicable, but not limited to Personal Computer Memory Card International Association (PCMCIA) wireless modems and Global System for Mobile Communications/General Packet Radio Service (GSM/GPRS) wireless networks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 10/305,634, entitled “Adaptive Current Limiter for Wireless Modem,”filed on Nov. 27, 2002, which issued as U.S. Pat. No. 7,106,569.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationstechnologies. More particularly, the present invention relates tocontrolling the current draw of a wireless Modem from a host powersupply.

BACKGROUND OF THE INVENTION

Integrated circuit cards (or “PC cards”) are often used in mobilecommunications and computing. In one particular application, the PC cardcomprises a wireless modem that plugs into a PCMCIA slot of a laptopcomputer or personal digital assistant (PDA). PCMCIA is an acronym for“Personal Computer Memory Card International Association”, which setsforth standards for PC cards. Typically, the host device (i.e. laptop orPDA) is coupled to a direct current to direct current (DC/DC) converter,which converts the host power supply voltage to a voltage source thatpowers the PC card.

For efficient use of the converted power by the wireless modem, theoutput impedance of the PC card must be well matched to the inputimpedance of the modem antenna. An impedance mismatch can occur if theantenna is not oriented for proper transmission, is broken or is incontact with an object that inhibits its ability to radiate radiofrequency (RF) power. A consequence of the mismatch is the transmissionof a weak RF signal and the possibility of the following two scenariosoccurring. First, the power control loop of the power amplifier (PA) inthe wireless modem will act to increase the PA output power tocompensate PA output loss due to antenna mismatch. Second, when the basestation of the wireless network receives the weak RF signal, the basestation will send a request for the modem to increase its output power,not knowing that the weak signal is attributable to the antennamismatch. Both of these scenarios may result in the PA of the wirelessmodem drawing excessive current from the host power supply, i.e., morecurrent than the supply is designed to properly supply. This phenomenonis often referred to as “current overdraw.” Current overdraw isundesirable since it can damage the power supply, cause the host deviceto reset, and/or cause the modem to overheat.

A solution to preventing current overdraw would be to use a fixedhardware limiter to limit the power amplifier DC power rail.Unfortunately, this solution has two problems. First, conventionalhardware current limiters only guarantee about a 20% threshold accuracy.This degree of uncertainty in accuracy may be too large to manage andprevent current overdraw by the power amplifier. Second, using a fixedhardware limit may result in the sending of a “false alarm” of a currentoverdraw condition to a host that may, in fact, have the ability todeliver a particular current draw demand (i.e. a “strong” power supply).Conversely, using a fixed hard limit may result in the sending of nowarning at all of a current overdraw condition to a host that may nothave the ability to deliver a particular current draw demand (i.e. a“weak” power supply). These problems would be compounded inmultiple-time-slotted systems, in which current demands vary dependingon the number of transmission slots allocated per transmission burst.

SUMMARY OF THE INVENTION

Generally, the methods and apparatuses of the present invention relateto wireless modems. More particularly, the methods and apparatuses ofthe present invention relate to controlling the current draw of awireless modem from a host power supply. The methods and apparatuses areparticularly applicable, but not limited to, mult-time slot GSM (GlobalSystem for Mobile Communications)/GPRS (General Packet Radio Service)networks.

According to one aspect of the invention, a current limiter for awireless modem includes a current sensor configured to measure a currentdrawn by a wireless modem from a power supply of a host device, acurrent-to-voltage converter coupled to the current sensor and operableto convert the measured current drawn by the wireless modem to a voltagerepresentative of the measured current draw, a reference voltagegenerator operable to generate a reference voltage defining a maximumcurrent that the modem may draw from the host device power supply, and avoltage comparator coupled to both the current-to-voltage converter andthe reference voltage generator operable to compare the voltagerepresentative of the measured current draw to the reference voltage andgenerate a current overdraw signal when the compare the voltagerepresentative of the measured current draw is greater than thereference voltage.

According to another aspect of the present invention, a method oflimiting the current draw of a PC card wireless modem from a host powersupply includes the steps of determining a current drawn by the modemfrom the host power supply, converting the measured current to a voltagerepresentative of the measured current draw, comparing the voltagerepresentative of the measured current draw to the reference voltage,and generating a current overdraw signal when the voltage representativeof the measured current draw exceeds the reference voltage.

Other aspects of the invention are described and claimed below, and afurther understanding of the nature and advantages of the inventions maybe realized by reference to the remaining portions of the specificationand the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an adaptive current limiting system,according to an embodiment of the present invention;

FIG. 2 shows an exemplary adaptive current limiter, which may be used insystem shown in FIG. 1, according to an embodiment of the presentinvention; and

FIG. 3 shows an exemplary current sensor circuit, which may be used inthe adaptive current limiter in FIG. 2, according to an embodiment ofthe present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a block diagram of an adaptivecurrent limiting system 10, according to an embodiment of the presentinvention. System 10 comprises a PC card wireless modem 100, whichincludes baseband and RF components, and an adaptive current limiter110. PC card wireless modem 100 includes a connector 102 for connectingPC card wireless modem 100 to a socket 104 of a host 106. Host 106 maycomprise a laptop computer, a handheld computer, a personal digitalassistant (PDA), or other device from which information (e.g. data) isto be radiated by an antenna 108 of wireless modem 100. As explained inmore detail below, adaptive current limiter 110 monitors theinstantaneous current of the power amplifier (PA) of wireless modem 100,via a PA sense line 112, and dynamically adjusts a current limitthreshold of the PA, according to, for example, the host power supplycapability, the number of transmission time (Tx) slots allocated pertransmission frame, and the RF band of the transmit signal. Adaptivecurrent limiter 110 may also be configured to send an over-currentsignal to wireless modem 100, via an over-current line 116, if forexample, the current draw of the PA exceeds an over-current level.Adaptive current limiter 110 may also send an error signal to a user ofthe system, via host interface 102, if for example, the antenna mismatchis detected by system 10. It should be pointed out here that whereassystem 10 shows the adaptive current limiter 110 and PC card wirelessmodem as comprising separate components, in an alternative embodimentthe current limiter would comprise part of the wireless modem and,therefore, not be a separate component.

Referring now to FIG. 2 there is shown an exemplary adaptive currentlimiter 20, which may be used in system 10, according to an embodimentof the present invention. Current limiter 20 is coupled to a DC/DCconverter 200, which converts a DC input voltage DCIN from a powersupply of a host device to a DC output voltage DCOUT. DC/DC convertersare known in the art and, therefore, will not be described in detailhere. DCOUT provides power to a PC card wireless modem, including thepower amplifier (PA) 201 of the wireless modem, as shown in FIG. 2. In aspecific exemplary embodiment, the host device power supply supplies aDC input voltage of 5 volts, DC/DC converter 200 is a buck typeconverter, which steps the DC input voltage down to an output voltageDCOUT having a nominal value of about 3.5 volts, and DCOUT is used topower a PCMCIA-compatible PC card wireless modem.

Current limiter 20 comprises a current sensor circuit 202, a voltagecomparator 204, a central processing unit (CPU) 206 (e.g. amicroprocessor, ASIC, or programmable controller), a digital-to-analogconverter (DAC) 208 and a current limiter shutdown circuit 210. Inaddition to accepting the over-current signal to decouple the powersource from PA 201 when the current source by PA 201 exceeds anover-current level, current limiter shutdown circuit 210 may providesecondary protection with a secondary, and possibly higher, fixedover-current threshold. A secondary fixed over-current threshold wouldbe used, for example, if CPU 206 is for some reason unable to respond tothe interrupt signal from comparator 204. This could happen, forexample, if the firmware on the CPU is out of control. A device, whichcan be configured to perform the primary and secondary protectionfunctions is part no. MIC2545A/2549A Programmable Current LimitHigh-Side Switch, sold by Micrel, Inc. Those skilled in the art willreadily understand that other circuits may be used to perform thesefunctions.

Current sensor circuit 202 detects the instantaneous DC current on theDC power rail of PA 201 and converts it to a voltage VOUT. VOUT iscoupled to a first input of voltage comparator 204 and is compared to areference voltage VREF, which is coupled to a second input of comparator204. Reference voltage VREF is generated by DAC 208 and has a value thatis adjustable by CPU 206. Other reference voltage generators may be usedand those skilled in the art would readily understand this. CPU 206 isprogrammed to provide a digital signal, which as explained above isconverted to an analog voltage by DAC 208, so that the VREF threshold isdetermined by one or more of the following: host power supplycapability, the number of present Tx slots and the RF operational band.The output of voltage comparator 204 provides an interrupt signal to CPU206, when VOUT is greater than reference voltage VREF, indicating thatthe instantaneous DC current drawn by PA 201 has exceeded apredetermined limit.

To enhance the accuracy of the adaptive current limiter circuitry, apre-calibration process may be performed during factory testing. Anexemplary pre-calibration process directs CPU 206 to program PA intodifferent constant current draw states. For each of these current drawstates, the DC current drawn by DC/DC converter 200 and the currentsensor 202 output voltage VOUT are measured and recorded. The measuredDC currents are then normalized and the VOUTs averaged. The normalizedDC currents and averaged VOUTs are then used to determine an offset ofthe current limiter under test. The offsets may be stored in anone-volatile memory and used as compensation offsets that can be addedto the reference voltage setting of voltage comparator 204. Whereas avoltage offset compensation table is described here, those skilled inthe art will understand that other compensation tables may be developedand used. For example, a temperature compensation table may also be usedto enhance the accuracy of the adaptive current limiter circuitry.

FIG. 3 shows an exemplary current sensor circuit 30, which may be usedfor the current sensor 202 in the adaptive current limiter in FIG. 2,according to an embodiment of the present invention. A sample(I_(SENSE)) of the current drawn by PA 201 produces a sense voltage dropV_(SENSE) across a sense resistor R_(SENSE). A mirror current I_(M)representing the sensed voltage is mirrored from a first differentialamplifier 300 to a second differential amplifier 302 via transistor 304and current mirror 306. Second amplifier 302 is configured so that itproduces VOUT, the voltage that is compared to VREF in FIG. 2. Othertypes of current-to-voltage circuits may be used for current sensor 202.Accordingly, those skilled in the art will readily understand that thecurrent sensor circuit 30 in FIG. 3 is but one of many possibilities.

In a specific exemplary embodiment of the present invention, theadaptive current limiter 10 in FIG. 1 (or 20 in FIG. 2) is used tomonitor and limit the current draw of a power amplifier in a PCMCIAwireless modem that is designed to conform to the multi-time slotGSM/GPRS communications protocol. In this particular embodiment, CPU 206is programmed to instruct DAC 208 to provide a reference voltage VREF,which is dependent upon either a two-slot or four-slot transmissionburst (i.e. class 10 or class 12 operation). Other parameters, such asfor example, the current supplying ability of the host power supply orthe frequency band of the signal transmitted by the wireless modem maybe used to define the reference voltage VREF. According to thisexemplary embodiment, the current draw threshold of adaptive currentlimiter 10 is adaptive to the one or more of the following conditions:(i) the type of host 106 that is being used, (ii) the number of TX timeslots being used, and (iii) the RF frequency band of which the modem 100is operating. The latter two conditions may change over time.Accordingly, a controller in the adaptive current limiter 110 (e.g. CPU206 in FIG. 2) may be configured to adjust the current limiter thresholdvoltage VREF dynamically based on changes in these conditions in realtime.

CPU 206 may be programmed to respond to an interrupt signal indicativeof a current overdraw condition in various ways. For example, inresponse to the interrupt signal, CPU 206 may be programmed so that anerror signal ERR is sent to a user, the error signal prompting the userto adjust the antenna of the PC card wireless modem. CPU 206 may also beprogrammed to provide a current draw reduction signal to an input of PA201 and/or may be programmed to produce a signal to reduce the number ofTx time slots used per transmission burst, in response to the interruptsignal. This response can protect the wireless modem from overheatingthe host power supply from being damaged or from resetting. Finally, CPU206 may also be programmed to send a signal to the user that the antennamay have been damaged or broken, if the modem continues operating in aprotected mode for an extended period of time.

In an alternative embodiment, if CPU 206 is capable of checking currentsensor 202 output VOUT directly at the beginning of each Tx slot, DAC208 and comparator 204 may be eliminated. According to this alternativeembodiment VOUT would be coupled directly to CPU 206 and ADC and CPUfirmware could periodically poll the PA current on, for example, everyTX time slot.

Whereas the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention, which is defined by the appendedclaims.

1. A method of limiting the current draw of a PC card wireless modemfrom a host power supply, said method comprising: measuring a currentdrawn by a PC card wireless modem from a host power supply; convertingthe measured current draw to a voltage representative of the measuredcurrent draw; comparing the voltage representative of the measuredcurrent draw to an adjustable reference voltage; and adjusting saidadjustable reference voltage as a function of one or more of host powersupply capability, number of transmission slots, and RF operational bandin order to establish a maximum permissible current that may be drawn bythe wireless modem from the host power supply.
 2. The method of claim 1wherein the maximum permissible current that may be drawn by thewireless modem from the host power supply is set depending on a currentsupplying capability of the host power supply.
 3. The method of claim 1wherein the PC card wireless modem comprises a PCMCIA compatible PCcard.
 4. A current limiter for a wireless modem, comprising: a currentsensor configured to measure a current drawn by a wireless modem from apower supply of a host device; a current-to-voltage converter coupled tothe current sensor and operable to convert the measured current drawn bythe wireless modem to a voltage representative of the measured currentdraw; a dynamically adjustable reference voltage generator operable togenerate a reference voltage defining a maximum current that thewireless modem may draw from the host device power supply, saidreference voltage being a function of one or more of host power supplycapability, number of transmission slots, and RF operational bands; anda voltage comparator coupled to both the current-to-voltage converterand the dynamically adjustable reference voltage generator operable tocompare the voltage representative of the measured current draw to thereference voltage and generate a current overdraw signal when thevoltage representative of the measured current draw is greater than thereference voltage.
 5. The current limiter of claim 4 wherein the voltagegenerator comprises: a central processing unit (CPU) having an interruptinput configured to receive the current overdraw signal and a digitaloutput; and an analog-to-digital converter (DAC) configured to receive adigital output signal from the CPU digital output and convert it to thereference voltage.
 6. The current limiter of claim 4 wherein the currentoverdraw signal is used to control the amount of current the wirelessmodem may draw from the host power supply.
 7. The current limiter ofclaim 4 wherein the wireless modem comprises a PCMCIA compatible PCcard.
 8. A wireless modem, comprising: a current sensor configured tomeasure a current drawn by the wireless modem from a power supply of ahost device; a current-to-voltage converter coupled to the currentsensor and operable to convert the measured current drawn by thewireless modem to a voltage representative of the measured current draw;a reference voltage generator operable to generate a reference voltagedefining a maximum current that the wireless modem may draw from thehost device power supply; and a voltage comparator coupled to both thecurrent-to-voltage converter and the reference voltage generatoroperable to compare the voltage representative of the measured currentdraw to the reference voltage and generate a current overdraw signalwhen the voltage representative of the measured current draw is greaterthan the reference voltage, wherein said reference voltage generator isdynamically adjustable during operation of the wireless modem to varythe reference voltage defining the maximum current that the wirelessmodem may draw from the host power supply, and is a function of one ormore of host power supply capability, number of transmission slots, andRF operational bands.
 9. The wireless modem of claim 8 wherein thevoltage generator comprises: a central processing unit (CPU) having aninterrupt input configured to receive the current overdraw signal and adigital output; and an analog-to-digital converter (DAC) configured toreceive a digital output signal from the CPU digital output and convertit to the reference voltage.
 10. The wireless modem of claim 8 whereinthe current overdraw signal is used to control the amount of current thewireless modem may draw from the host power supply.
 11. The wirelessmodem of claim 8 wherein the wireless modem is PCMCIA compliant.
 12. Themethod of claim 1, further comprising generating a current overdrawsignal when the voltage representative of the measured current drawexceeds the adjustable reference voltage.
 13. The method of claim 12wherein the current overdraw signal is used to control the amount ofcurrent the PC card wireless modem may draw from the host power supply.